1. Field of the Invention
This invention relates to a process of forming a coherent refractory mass on a surface by projecting against that surface, together with oxygen, a mixture of refractory particles and fuel which reacts in an exothermic manner with the projected oxygen to release sufficient heat to melt at least the surfaces of the refractory particles and thus form said refractory mass. The invention also relates to a mixture of particles for use in a process of forming a coherent refractory mass on a surface by projecting against that surface the mixture and oxygen, said mixture comprising refractory particles and fuel particles which are capable of reacting in an exothermic manner with oxygen to release sufficient heat to melt at least the surfaces of the refractory particles for forming said refractory mass.
2. Description of the Related Art
If it is desired to form a refractory mass in situ on a surface, there is a choice between two types of known process.
In a first type of process, sometimes referred to as "ceramic welding" and illustrated by British Patent No. 1 330 894 (Glaverbel) and British Patent application published under No. GB 2 170 191 A (Glaverbel), a coherent refractory mass is formed on a surface by projecting against the surface a mixture of refractory particles and fuel particles, together with oxygen. The fuel particles used are particles whose composition and granulometry are such that they react exothermically with the oxygen to result in the formation of refractory oxide and release the heat required to melt at least the surfaces of the projected refractory particles. Aluminium and silicon are examples of such fuels. Because silicon behaves like some metals in that it is able to undergo a strongly exothermic oxidation to form a refractory oxide, although we recognise that silicon should properly be considered as a semi-metal, it is convenient to refer to these fuel elements as metallic. In general it is recommended to project the particles in the presence of a high concentration of oxygen, for example using commercial grade oxygen as carrier gas. In this way a coherent refractory mass can be formed which is adherent to the surface against which the particles are projected. Because of the very high temperatures in the ceramic welding flame, the flame tends to be able to cut through any slag which may be present on the surface of a refractory being treated and soften or melt that surface, so that a good joint is made between the surface being treated and the newly formed refractory mass.
Such known ceramic welding processes may be used for the formation of a refractory element, for example a block of special shape, but they are most commonly used for forming coatings or repairs on refractory blocks or walls, and they are particularly useful for repairing or reinforcing existing refractory structures, for example for repairing walls or wall-coatings of glass-melting furnaces, coke ovens or refractory equipment used in the metallurgical industries. It is usual to effect such an operation while the base refractory is hot, and in some cases it is even possible to effect the repair or reinforcement without interrupting normal operation of the equipment.
It is evident that the efficient working of such ceramic welding processes requires rapid and complete release of the heat liberated by the reactions between the fuel particles and the oxygen. In other words, it is desirable that all the fuel particles will have been completely burnt before they reach the surface being sprayed. Also, the high cost of suitable metallic fuel particles will encourage the ceramic welder to obtain the maximum yield, that is to say to work so that the combustion of the fuel is as complete as possible and no residual unburnt fuel is occluded in the refractory mass formed.
The second type of process for forming a refractory mass in situ on a surface is known as flame spraying. Such processes consist in directing a flame over the location where it is desired to form the refractory mass and projecting refractory powder across the flame. The flame is fed by a gaseous or liquid fuel, and sometimes by powdered coke. It is evident that efficient operation of such flame spraying techniques requires the complete combustion of the fuel to create as hot a flame as possible and obtain the maximum yield. In general, the flame temperature which can be obtained in a flame spraying process is not as great as can be achieved in a ceramic welding technique, with the result that the coherence of the refractory mass formed is not so great, and since the joint between the new refractory mass and the surface of the refractory base is formed at a lower temperature, that joint will not be so secure. Such a flame is much less able to penetrate any slag that may be present on a refractory surface being treated than is the flame of a ceramic welding process.
Ceramic welding and flame spraying technique such as have just been described are useful for facing or repairing walls or coatings constituted by various classical refractory materials, such as basic, silica, silico-aluminous and zirconiferous refractories.
Nowadays, there is an increasing use of refractories of a new type which is characterised by a high content of carbon particles. These carboniferous refractories are usually based on magnesia or alumina, and they may contain from 5% to 30% or even 35% carbon by weight. Such carboniferous refractories are used in industrial electric melting furnaces and also, in steelworks, in convertors and casting ladles. They are chosen for the high resistance to erosion and corrosion by molten metals and slags.
When facing or refacing a refractory structure, it may be desirable to form a refractory coating having a better resistance to erosion and corrosion than the base material. This is especially the case at parts of the refractory structure which are particularly liable to the effects of molten material, such as the pouring spouts of casting ladles. More usually however, and when repairing a refractory structure, it is preferred to form a refractory mass which has the same composition as the base material. This helps to ensure that the new material is compatible with the base material on which it is formed both as regards its chemical composition and its expansion characteristics. If there is chemical or physical incompatibility between the new and old refractory materials, the joint between them tends to be poor, and the repair or facing can flake off. Thus there is a requirement to be able to form coherent, compact (that is, non-porous) refractory masses which have the same or a closely similar composition to that of the carboniferous refractories referred to above and which will adhere well to a surface of a given refractory material.
Given the requirement for forming a carboniferous refractory mass, it would appear necessary that this must be done at a temperature which is not too high, or under conditions which are not, or are only slightly, oxidising. Thus it would seem appropriate to make use of a flame spraying technique as described above, spraying a mixture of coke and refractory particles under conditions such that there is insufficient oxygen for complete combustion of the coke. An alternative method would be to apply a paste of the required composition and fire it en masse.